Purpose is to align the mold and the stationary platen on the machine.
Generally centered on the mold but not necessarily.
Extended versions are available with extra length and lead-ins to make alignment easier.
Fattori, Jim. “What You Need to Know About Locating Rings: Part 1.” Plastics Technology, www.ptonline.com/articles/what-you-need-to-know-about-locating-rings-part-1.